Known in the art is a process for preparing a film-forming composition by way of co-oligomerization of arylalkenes, such as styrene, with conjugated dienes, such as butadiene, at a weight ratio of arylalkenes to dienes of 1:4, temperature of 150.degree. C. in a medium of a hydrocarbon solvent such as toluene, ethylbenzene, cumene or xylene taken in a 3.5-fold excess with respect to the mixture of the starting monomers, in the presence of a co-oligomerization initiator, i.e. an organic peroxide such as tert.butyl peroxide, or an organic hydroperoxide. Said hydrocarbon solvent also acts as a molecular weight control agent. From the resulting co-oligomerizate the hydrocarbon solvent and unreacted monomers are distilled off. The residue comprises a liquid low-molecular (molecular weight of from 500 to 4,000) co-oligomer of an arylalkene with a conjugated diene; the co-oligomer yield is 80% by weight of the mixture of the starting monomers. To prepare a varnish (i.e. film-forming composition) on the basis of said co-oligomer, the latter is dissolved in benzene, chloroform or dioxane.
This prior art process has a disadvantage residing in the use of toxic hydrocarbon solvents in both co-oligomerization and in the varnish preparation, wherefore the range of application of said varnish becomes limited.
Another disadvantage of the prior art process resides in the use of an excess of the above-mentioned hydrocarbon solvent with respect to the starting monomers in the co-oligomerization, wherefore the concentration of the monomers in the reaction mixture is reduced and, consequently, the rate of the co-oligomerization process is lowered.
Furthermore, the prior art process is technologically complicated, since it necessitates distilling-off the hydrocarbon solvent such as cumene from the co-oligomerizate and dissolution of the resulting co-oligomer in another toxic hydrocarbon solvent such as benzene or chloroform.
In the prior art process a high conversion degree of the starting monomers is not achieved. This results in a intolerably high content of the unreacted toxic arylalkene, e.g. styrene, in the resulting co-oligomer which, in turn, increases the toxicological properties of the varnish produced from said oligomer, wherefore the range of application of said varnish is restricted.
Owing to the use of the above-mentioned hydrocarbon solvents simultaneously serving as molecular-weight control agents in the co-oligomerization process, the resulting co-oligomers contain, as the terminal groups, fragments of the solvent with non-reactive functional groups which slow-down the process of oxidative polymerization (curing) of oligomeric films.
The use of conjugated dienes in an excess with respect to arylalkenes results in a predominating content, in the oligomeric chain, of units of conjugated dienes (compared to the arylalkene units). This causes poor drying properties in a film-forming composition (varnish) produced from such oligomers and poor adherence and weather fastness of a coating made of said composition. Due to poor drying properties, the film-forming composition based on said oligomers can be used only as a hot-drying varnish.